Hoffman 706 SOLAR 1960's
 

Hello,
Here's a project I've been waiting over 40 years to start. My Dad bought this in the '60s and played it all the time.. One fateful day when it was kinda dark outside due to a snow storm, Dad put the radio up on a pole lamp upside down so the radio would collect light from the lamp.
It melted the solar panel and part of the case.. durn! That was the last sound it made.. :sigh::tears:

I've been searching for a new solar panel. I've tried to take the old one apart to solder those little wires. It's just laminated too much to get apart.

http://www.rsgpianoservice.com/file/hoffman_solar.JPG

My radio is black and this (above) is just another pic I found of the same model. It's a cool radio. These solar panels were made by the same company that made solar arrays for some of the early satellites!

I've made several inquires into finding a replacement NOS or even a used solar panel for this radio. Any ideas? Maybe an old repair shop might have one in the back room setting there on the top shelf in an old box gathering dust. (grin) :D

These radios are "trying" to sell for around 150 bucks on FreeBay. That's too much for a solar panel and a parts radio for me. Me cheap ol' dude!

Thanks and Happy Thanksgiving!

Buzz

Check out American Science Surplus:
http://www.sciplus.com/search.cfm?ut...=0&btnHand.y=0

Tanks
 

Thanks, Mr. Nut...

I'll have to write them.. the ones posted of course, aren't very close to this one..

I'll take some pics of mine for "show and tell".

Buzz

Pix of my Hoffman
 

Howdy Guys 'n' Gals,

Here's some pics of my little Hoffman. I notice on FreeBay one of these selling for around 190.00 or so.. Wow..!

I've received some replies to my solar panel inquiries but no luck yet.

http://www.rsgpianoservice.com/file/solar1.jpg

http://www.rsgpianoservice.com/file/solar2.jpg

http://www.rsgpianoservice.com/file/solar3.jpg

http://www.rsgpianoservice.com/file/solar4.jpg

I've noticed that there's two types of speaker grills. Mine is the smooth type and the other has ridges in it. All the solar panels look the same though.

Buzz:music:

You might not want to but wonder if you could adapt some other solar cells currently (sorry!) available to it? What total voltage does it need?

Adapt
 

Hi..

Just 4 volts.. I'm trying to keep it "original"..

http://techpreservation.dyndns.org/s...n_P706_69.djvu


But.. I had thought of that. Above is the link to the schematic.

Thanks,

Buzz

My guess is that the cells on your Hoffman are quarter rounds from a small diameter ingot; and therefore you won't find any of the same shape unless they are from the same era. You might consider newer rectangular ones just to get it going while you search for the originals.

Do you have the radio itself working? If so, have you measured the current draw at 4 volts? The original solar array drops to around 3 volts at 12ma of current draw, so minimizing current draw is an absolute must in order to get these things to run from the solar array.

After you replace the 5 electrolytics, you'll probably have to replace R14, R15, R16, R17 and R21. Although it doesn't say anything about this in the Sam's, I believe some of these resistors, if not all 5 were hand-picked or at least sorted at the factory prior to assembly. This is/was necessary to get the operating current down to 7.5ma no signal/minimum volume, 10 ma with signal/normal listening volume. All 5 of these resistors work together to set up the bias of the 2 2N185 push-pull output transistors.

Every one of these sets that I have worked on have required the replacement of these resistors. If you simply replace the electrolytics, you'll probably find that the operating current is a flat 16 to 19 ma., with more than 85% of that due to current draw by the audio output stage due to incorrect biasing. On batteries or a power supply, it will function perfectly that way! But it will NEVER run from the solar array with that much current draw. A quick test to see if the bias is correct is to measure the voltage drop across R20. It should be around .038 to .04 volts at normal listening levels.

Fun fact: Minus the audio output stage, this set draws less than 2.5 ma, including the audio driver stage! That's less than 1/100th of a watt! Not bad for 1958 technology!

I know that this doesn't help you with your search for a replacement solar array, but when you get to that point, it may keep you from pulling your hair out!

Good luck,
Matt

Thanks
 

Hello,

Thanks for the great advice.

I haven't done anything to this yet. I've just been waiting for a Panel for it... then I'd jump into it.

I've got a power supply I can turn down to the 4 volts so I'll see what I can find.

Buzz :music:

The good news is that if you find one that doesn't appear to have any physical damage, with the seal on the back still intact, it will in all likelihood work fine.

I personally haven't seen a bad one yet.

Update
 

:yes:

I ended up cutting the back of the Solar panel plastic off.. I just traced around and around with an Xacto knife until it popped off. What I found as the cause of the Solar panel not working was the wires leading from one solar "chip" to the next were either un-soldered or cold soldered. Leading me to believe that my Dads setting the radio upside down on top of a lit lamp caused the solder to melt. I re-soldered all the connections lightly and the panel came back to life!

I then changed out all the Ecaps and the radio works! After 45 years.. it's making music again!:banana::banana::banana::banana::banana:

Nice!

Buzz:music:

Cool! Glad you were able to repair the array and get the radio working.

BTW, I have a solar array from a later Hoffman (709X) that puts out a lot more power with much less light. With that one connected to a 706, the radio will play full blast under the light of a 60 watt lamp more than 2 feet above it. But since it's a 6 volt array, you wouldn't want to expose it to the sun while connected to a 706.

solar shmoler
 

I'm glad I got the "gumption" to go ahead and tear it apart. I've had this little radio with me sense I was a kid in the 60's.

Next, after a couple of other projects here on the bench, I think I'll test some of the resistor values. The volume is a little low and that is just the solar panel, but the sensitivity is not very good. I should be able to pick up more stations than the close one in town (20 miles away)!!

So the project lives on. Oh.. I did peak the rf and If coils.. to no avail.

Still like to find a junker (black in color to match) so I could change out the back cover. (see pics above) It's warped. Front looks nice after I cleaned it up.:thmbsp: These things are really over priced on Efray!.. Wow what a racket! Efray has really inflated all things us hobbyists like to collect. But on the other hand.. Efray allows one to cover a lot of turf by bringing many togeather for a common cause.

Buzz :music:

Glad to hear you got it working again! Never saw a Hoffman version of this set. I have seen several Zeniths from the same era with solar panels, although they put the panel in the handle, making it look pretty terrible.

Yes, the Hoffman 706 series is a bit more elegant, with its solar array built into the case. It also pre-dates the Zenith Sun-Charger by 6 or 7 years.

Well, I finally came across a Hoffman 706 with a non-functioning solar array.

This one is a TP706, and although it looks good, it's a basket case.

First off, the speaker cone is water damaged. The radio doesn't appear to have been in a flood, so my guess is it was probably left under a sprinkler at some point in its life. Second, the solar array didn't have any output. This is one of the early 706's with the rectangular solar chips that are soldered together in an overlapping fashion like shingles. I cut the adhesive away from the bottom panel to remove it and found that the foam that held the array against the top of its enclosure had completely deteriorated to mush, allowing the array to move around and break into 4 pieces. The other issue can be seen if you look closely at the area where the negative wire is taped to the back of the array. Apparently the adhesive of the tape, in conjunction with the deteriorating foam, caused the insulation of the wire to completely break down. The wire is shorted to the back of panel.

This solar array is extremely fragile! It's going to be a challenge to solder it back together!

http://www.smokypond.com/gallery/dow...ray-Broken.JPG

Solar pan
 

Looks like a project.

When I soldered my panel pieces back togeather I turned down my soldering heat Temperature. What I wished I had was some silver solder. Something that would melt at a lower temp. I noticed that the panel material has a real low melting point, so be careful.
My spongy stuff was in bad shape too. I couldn't find anything to replace it with. What were you going to use to replace the sponge with?

Good luck

Buzz:music:

I may try to use some of that low-temp (430 F) solder paste that comes in a syringe. I'll be careful though since I'm not sure how it will react with the original solder and silicon.

For the foam, I know I have something at home or at the office that will work. I'll just have to make sure that it's not the anti-stat variety!

Make sure it's not Conductive.. yes.. that would be anti-static.

Some anti-static bags have a Detergent coating that conducts the static around to the other side or equalizes it. I don't know about bug (for IC's) foam or sponge.

Buzz:music:

Success! Got the array put back together. It puts out about 12ma under the light of a 60 watt bulb. That's as good as gold for one of these early solar arrays.

The solder used on this array was not low-temp by any stretch of the imagination. My little Hakko iron would not generate enough heat to melt the solder to sweat the joints back together. It was like trying to melt aluminum. I ended up using a Weller micro-torch. Worked great and it didn't damage anything. While I was at it, I touched up all the joints, since quite a few of them exhibited some flex. Afterwards the whole thing was good and solid, and flat.

I could not find the correct thickness of black foam for the backing, so my wife Janet took a 1/2" thick piece of extremely soft small-celled black foam and cut it down to 1/8" thickness with a wide blade X-Acto knife (she ALWAYS helps during a restoration, whether it's a radio, TV, tape-deck, old truck or motorcycle). I put the back on and sealed it with GC Vinylite cement, which appears to be the same stuff Hoffman used to close it up.

Now on to the radio and speaker.....

http://www.smokypond.com/gallery/dow...olar+Array.JPG

Your tourch
 

Wow..

That's great! Looks brand new!

It's neat that da Spouse helps out too.! Next ... she'll be buying old radios for herself to restore.. Then she'll become a ham. Say good buy to your nice shop area. heehee!

Buzz:music:

I forgot to mention that I did tack solder some new electrolytics into this one just to find out how close it is to working. It seems to show signs of low sensitivity like you mentioned. After I install the caps permanently, I'm going to hook it up to the signal generator and scope to do a side-by-side comparison with one of my other 706's to try to determine the cause. It may need some tweaking, but it didn't look like anyone had been inside this one. Hopefully it's not a tin whisker problem in one of the transistors.

One thing I did notice though is that the loop stick antenna in this one is different than my other 706's. The windings on the ferrite bar go from one end to the other. There is just barely enough room for the 2 little metal clips that hold the bar to the circuit board. My other 706's have windings that use just 3/4 of the bar length.

I'll post my findings when I get there.

The PC board is different too. Looks a lot older and doesn't even come close to what's shown in the Sam's.

I worked on it for awhile, then my signal generator quit. Transformer! Crap! Bought a new "modern" one on eBay. Should be here tomorrow. But what I've found so far is that this ain't gonna be easy!

The 2N253 1st IF transistor is bad in my TP706. As I mentioned earlier, the set plays, but suffers from low sensitivity and volume.

BTW, the transistor date codes are all 830 or 831. That equates to the 30th and 31st week of 1958.

Not sure what I'm going to replace it with yet.